The quality of paper or board can be improved by, among other ways, coating the paper or board web with a suitable coating mix. The coat is most commonly applied using brush applicators, nozzle applicators and for short-dwell coaters, all of which are combined with an almost simultaneous control of coat thickness by means of a doctor blade. These methods all employ a narrow slit formed proximate to the moving web, whereby the amount of applied coat is controlled by the slit. The final control of the coat weight may be performed in a number of different manners. Using the above-mentioned methods under conditions optimized for each method, a good coat quality can be attained at relatively high web speeds. However, running the web at a speed above a certain limit may be difficult due to a number of reasons. During application, the web is subjected to a relatively severe strain, which, particularly at high web speeds, readily causes web breakages. During coat application, the web is slightly elongated by the tensional strain and increased moisture content, whereby such elongation must be managed by the drive control system of the paper machine. Another shortcoming of conventional coaters in application at high web speeds is the uncontrolled flow of the coating mix at the instant of application resulting in uneven coat smoothing, which can be seen as blots and streaks on the web and as splashing around the machine. The severe strain imposed on the web during doctor blade coating causes a coating problem.
Because industry demand is for paper making machinery operating at higher web speeds, novel methods must be developed for the application of coating mix onto the web surface. One of these new methods is spray-coating in which the coating mix is blown from spray nozzles onto the web. One form of spray-coating is ultrasonic coating in which the coating mix is atomized into small droplets with the help of ultrasonic energy and the droplets are guided to hit the web surface chiefly ejected by the ultrasonic atomizer. Alternatively, ultrasonic energy can be used merely for atomizing the coating mix into droplets. The principal benefit of ultrasonic coating is that an extremely small droplet size can be achieved, whereby the applied coat becomes homogeneous and no mechanical doctoring gap for coat weight control is required in the vicinity of the web. The ultrasonic nozzle has a nozzle channel ending at the nozzle tip surface, and the material to be atomized is fed into the channel. Piezoelectric elements are adapted about this tubular part, emitting ultrasonic vibrations capable of atomizing the material to be spray-coated into an aerosol.
Wider use of ultrasonic coating has been limited by the lack of sufficient atomizing output power in commercially available ultrasonic nozzles. For instance, if it is desired to coat the web with an amount of coating mix that results in 10 g/m.sup.2 dry coat weight, the amount of coating mix applied onto each linear meter of a web running at 30 m/s must be 600 gs.sup.-1 m.sup.-1 when the coating mix contains 50% solids. Then, the required ultrasonic atomizing effect is 1000-3000 gs.sup.-1 m.sup.-1. However, the output power of conventional ultrasonic nozzles is so low that the required number of nozzles readily becomes very high if the web speed and amount of applied coat is increased. This type of nozzle makes it difficult to achieve high-speed application onto a narrow area which would be advantageous in terms of minimal moisture absorption into the web. Nozzles with a circular orifice are also hampered by their spot-shaped hit area of the atomized droplets which makes it difficult to achieve a homogeneous coat as the hit areas of the nozzles cannot be merged with each other in a seamless manner. Difficult-to-pump mixes, for example, high-viscosity coatings are also difficult to atomize by means of conventional ultrasonic nozzles.